It is often desirable to be able to provide an indication of whether a product has been exposed to undesirable temperature history or UV exposure. This applies to perishable goods such as foods, pharmaceutical products or photographic products which have a limited lifetime that may be shortened by exposure to extreme environmental conditions.
A variety of devices are available to provide this information. These may be divided broadly into two classes, those that contain electronics to enable sensing, display and data recording, and simpler devices which contain no electronics. Whilst the cost of electronic components is continually being reduced due to economies of scale and mass manufacture, for many packaging applications the cost of the device and the integration required is still prohibitive. Of the second class of device, which contains no electronics, several examples exist. For instance US 2006/0011124 describes a temperature sensitive label that may be fixed to a package. The label has a mixture containing a petroleum wax that melts at a prescribed temperature. Upon melting, the wax penetrates into a coloured layer making it transparent and so allowing a coloured face beneath the coloured layer to become visible. A similar device that may be incorporated onto a package is described in JP 6050827A. This document discloses a hot-melt substance encapsulated within a pressure sensitive blister that may be depressed to allow the hot-melt substance to come into contact with an absorbing medium. The absorbing medium has a pattern written above it such that after pressing the blister, when a prescribed temperature is reached a message can be displayed upon the package. A similar device, described in U.S. Pat. No. 7,232,253, uses a migration medium of controlled porosity to measure elapsed time by the ingress of a coloured liquid into the migration medium. By changing the porosity of the medium and the viscosity of the coloured liquid the device can be used to measure different time periods. The device is incorporated into a self-adhesive label that may be affixed to items of food to give a visual indication when the item has reached the expiry date.
U.S. Pat. No. 6,741,523B1 describes a time dependent indicator formed using a complex microstructured substrate in which fluid is separated from the microstructured substrate until the indicator is activated, at which point the fluid progresses through the channels of the microstructured substrate at a known rate. Fluid may be separated by a physical barrier which can be ruptured at the time of activation to allow the fluid to come into contact with the microchannels.